The invention relates to a device for determining a desired contour of a ground surface, for instance a road surface, relative to the actual contour of that ground surface, at least comprising measuring means for generating along a determined distance from a ground surface actual values for the height and position of a number of points of that ground surface relative to a determined reference point.
Such a device for determining the contour of a road surface is known for the purpose of applying a new top layer of for instance asphalt or asphalt concrete, wherein use is made of a so-called multi-foot ski. A multi-foot ski comprises a number of flat plates or feet, two of which are mounted pivotally in each case on an elongate first arm which, using a second elongate arm, is in turn pivotally coupled at a point in the middle to a following first arm on which two feet are also pivotally mounted. Using a third elongate arm the second elongate arm is pivotally coupled at a point in the middle to a corresponding subsequent second arm, wherein all arms extend in the same direction. During asphalting operations a multi-foot ski is dragged by an asphalt spreading machine alongside this machine over a strip of road surface adjacent to the part of this road surface which is to be asphalted. The height of the middle of a first arm of the multi-foot ski relative to a measuring point on the asphalt spreader is determined by the average height of the respective feet under this first arm. The height of the middle of a second arm of the multi-foot ski relative to the measuring point on the asphalt spreader is determined by the average height of the middles of the respective first arms under this second arm. The height of the middle of the third arm of the multi-foot ski relative to the measuring point on the asphalt spreader is determined by the average height of the middles of the respective second arms under this third arm. Situated on the asphalt spreader at the measuring point is a height sensor, for instance an ultrasonic transmitter/receiver combination, which measures the height of the middle of the third arm and thereby the average height of all feet in the multi-foot ski. In this way an average value progressing in lengthwise direction of the road surface is obtained for the height of this road surface, wherein abrupt transitions in height, resulting for instance from damage to the road surface, are averaged out. This average value is entered into a control circuit for a finishing beam which is dragged along over a newly laid asphalt layer in order to give this layer the desired thickness, wherein the effect of discontinuities in the underlying road surface is reduced by averaging.
U.S. Pat. No. 4,065,856 discloses an optical system for a reference datum in relation to which operations can be carrried out in roadway workings. This optical system comprises optical devices which define an optical triangle whose plane inclines downwardly towards the plane of the roadway. The inclination of the plane of the optical triangle is adjustable so that a corner of the triangle is set at the level of the roadway as determined by a reflected image of the roadway surface. The optical system comprise a light source, e.g. a laser, and a receiver, which both are carried by a table which is pivotally mounted on a roadway maintainance machine, in this case a conventional track lifting and ballast tamping machine. The table is mechanically coupled for vertical movement with a track lifting device. The emitted light beam reflects on the underground of a railway track and is swept over the underground by pivoting the table, until the reflected beam just falls in the middle of the detector. In this situation the lifting device is at its desired height to perform its function, after which the machine is moved forward and the described operation is repeated.
According to this prior art, the reference point for levelling a railway track is situated on a fixed distance in front of the machine. Modulations in the contour of the ground surface are thus reduced by a factor corresponding with the ratio of distances from the pivoting point to respectively the reference point and the point on the ground surface to be corrected.
According to the prior art it is not possible however to eliminate the effect of modulations in the ground surface, making this device not applicable in combination with e.g. an asphalt spreader, a grader or a scarifier.
DE-A-44 27 724 discloses a device for performing a trigonometric measurement of an angle-dependent variable, comprising a continuous laser, emitting its coherent light on a rotating mirror. In the range of the pivoting laser beam two detectors are provided: a reference detector and a distance detector, the signals of which are to be compared in a comparator, for calculating a rotation angle from the time differences.
DE-A-196 24 751 discloses a device for performing a transit time measurement with a continuous laser, emitting its light on a rotating mirror. The rotating mirror projects the laser beam on a position resolving detector for generating a signal from which transit time information is to be obtained.
The known contour-determining device with multi-foot ski has a number of drawbacks. Due to the mechanical suspension of the feet the measurement range of the measuring device which can be realized in practice is smaller than is desirable for a good insight into the contour of a road surface. Transport and assembly of the multi-foot ski are time-consuming, while during the disassembly required in respect of transport components are in practice quite often lost. Use of the multi-foot ski in a contour-measuring device is limited to a ground with a solid structure due to the required mechanical contact with a ground, so that the multi-foot ski is for instance unsuitable for measuring the contour of a sandy ground. In an asphalt spreader the multi-foot ski must necessarily always be placed just adjacently of the part of a road surface for asphalting so that only an approximation of the contour of the road surface for asphalting can be obtained. In practice the feet of the multi-foot ski are nevertheless found during asphalting to come into contact with and to be fouled with a freshly applied tacky layer, which may result in poor functioning and in any case requires additional maintenance.
The object of the invention is to provide a contour-measuring device which does not have these drawbacks.
This objective is achieved and other advantages gained with a device of the type stated in the preamble wherein according to the invention the measuring means comprise: a laser light source which can be placed at a measuring point above the ground surface and which is adapted to generate a laser beam moving over said determined distance from the ground surface, sensor means for detecting the laser beam reflected by the ground surface to the measuring point, time-measuring means for determining the transit time of said laser beam between the laser light source and the sensor means, in addition to processing means for generating from signals from the laser light source, the sensor means and the time-measuring means of actual values for the height and the position of a number of points of this ground surface relative to the measuring point, and for calculating from these actual values of values for height and position of a number of points on a desired contour of this ground surface.
The laser light source generates for instance a pulsed laser beam scanning a determined angular range which strikes the ground surface, is reflected and detected by the sensor means, wherein the transit time is directly proportional to the distance between light source and point on the ground surface. The position of the point on the ground surface can be precisely determined relative to the laser light source (the measuring point) from the angle at which the laser beam was transmitted and the determined distance.
With a contour-measuring device according to the invention it is possible to measure the contour of a ground surface in contact-free manner and to determine for each point on this ground surface a desired height which is determined by the actual height of that point and of a large number of nearby points on that ground surface, wherein the processing means correct for discontinuities in the values for the actual height of these points.
It is possible with a measuring device according to the invention to determine the desired contour on the basis of the actual height of a very large number of points in close succession over a relatively large range on the ground surface, while the number of points for determining an average height of a point using the known measuring device with a multi-foot ski is limited to the number of feet under this ski, wherein the total length of the ski moreover defines the range of the measurement.
In a contour-measuring device according to the invention the processing means are preferably adapted to replace an actual value for the height of a point with a substitute value if this actual value exceeds a predetermined threshold value.
With this preferred embodiment it is possible to determine the contour of a road surface for asphalting within the width range of an asphalt spreader, since incorrect information concerning the actual contour of the road surface, which can for instance result from reflection of the scanning laser beam on components of the asphalt spreader, are recognized and eliminated.
In an advantageous embodiment the processing means are adapted to separately calculate for each of the number of points on a desired contour of the ground surface the values for height and position of these points. The possibility of calculating for each of a discrete number of points on a desired contour the height relative to the corresponding actual point makes it possible, otherwise than in contour determination with a multi-foot ski, to make the desired contour while taking into account the occurrence of a variation over a short distance in the actual contour relative to the average contour measured over a longer distance. In particular it is possible to design a desired convex contour portion above an actual concave contour portion in a road surface. In a practical situation such a concave contour portion results from settling of the ground at that location. This settling of the ground can be taken into account during asphalting by depositing the asphalt layer in accordance with the designed convex contour, whereafter the asphalt undergoes a so-called postcompaction with the action of a roller thereover.
In yet another embodiment the contour-measuring device according to the invention comprises memory means for storing values for the actual position of a number of predetermined points of a ground surface and the predetermined value for the desired height of the-se points, and the processing means are adapted to calculate the value for a desired height of points located between these predetermined points compatible with this predetermined value for the desired height of these points.